1. Technical Field
The present invention relates generally to a system and method for minimizing cure-induced residual stress in an epoxy resin impregnated apparatus, and more particularly, to an epoxy resin impregnated ignition coil, in one embodiment.
2. Discussion of the Background Art
Ignition coils are known for use in connection with an internal combustion engine, such as an automobile engine, and which include a primary winding, a secondary winding and a magnetic circuit. The magnetic circuit conventionally may comprise a cylindrical-shaped, central core extending along an axis and located radially inwardly of the primary and secondary windings and magnetically coupled thereto. These components are contained in a case formed of electrical insulating material, and may optionally include an outer core or shield located outside of the case. It is known to introduce an encapsulant, such as an epoxy resin, into the ignition coil, as seen by reference to U.S. Pat. No. 6,178,957 entitled xe2x80x9cPENCIL IGNITION COIL ASSEMBLY MODULExe2x80x9d issued to Widiger et al. Widiger et al. further disclose that the encapsulant is introduced in liquid form and is allowed to flow into the interior of the primary bobbin and fill an annular space surrounding the secondary bobbin and secondary coil, and is further introduced to bring the level sufficient to fully cover the components. Widiger et al. further disclose that the material is allowed to cure. However, there are shortcomings associated with the above-described conventional encapsulation/curing approach. One problem is that when the encapsulant (e.g., epoxy resin) is cured, chemical shrinkage occurs, leaving voids, and resulting in small gaps at critical interfaces. In context, it should be understood that for an ignition coil, the epoxy protects hair thin wires, providing both electrical insulation, as well as environmental isolation from environmental factors that the ignition coil may encounter during its service life. By way of example, as shown in FIG. 5, a conventional approach for curing the epoxy resin involves a two-step process, which is shown as trace 102. That is, in a first step, the temperature (ambient) is brought up to a first level and is held for a predetermined time. The next step involves raising the ambient temperature to a second, higher level. It is held at this second temperature for another, second predetermined time. The resulting residual stress in the epoxy, which is shown as trace 104, is substantial, particularly during the xe2x80x9csecondxe2x80x9d step increase in temperature. The rapid increase in the tension represents interfacial stress, which results in a loss of adhesion between the epoxy resin and the components it is intended to encapsulate, for example, a secondary winding wire. A separation may result in microarcing during operation of the ignition coil, which can eventually break down the materials in the ignition coil, causing it to short and fail. Therefore, the cure-induced stress, in conventional encapsulation/curing approaches, results in a less than optimal outcome.
In the art, it is known to provide a closed loop feedback system which employs thermal expansion to counteract the stresses resulting from chemical shrinkage, as disclosed in an article entitled xe2x80x9cA NEW METHOD TO REDUCE CURE-INDUCED STRESSES IN THERMOSET POLYMER COMPOSITES, PART II: CLOSED LOOP FEEDBACK CONTROL SYSTEMxe2x80x9d by Genidy et al., Journal of Composite Materials, Volume 34 (hereinafter referred to as the Cure Induced Stress Test or xe2x80x9cCISTxe2x80x9d article). The CIST article discloses an apparatus that measures strain on a thin carbon fiber produced through a gel-cure cycle of an epoxy resin. The apparatus includes closed-loop feedback to reduce the cure-induced stress during the gel-cure cycle. However, the CIST article assumes that the epoxy resin that is being cured is isothermal throughout its volume, and is equal to the surrounding ambient temperature. This is a significant limitation, however, inasmuch as most practical encapsulation/curing applications do not conform to the isothermal model used in the CIST article. Accordingly, simply following the teachings of the CIST article would not result in minimizing stress under non isothermal conditions.
Another shortcoming with conventional encapsulation/curing approaches involve the relatively lengthy times required for curing the epoxy resin. In a production scenario, increased cure times result in decreased throughput and productivity, and may become bottlenecks.
There is therefore a need for an improved system that minimizes or eliminates one or more of the shortcomings set forth above.
An object of the present invention is to solve one or more of the problems as set forth above. A method and apparatus according to the present invention overcomes the shortcomings of conventional encapsulation/curing approaches to provide an optimal stress profile during the gel/cure cycle, thereby reducing or eliminating voids and separations between the epoxy resin and the component being encapsulated, and, further yielding stronger adhesion therebetween. Because of these improved characteristics, product failures resulting from the failure mode of epoxy separation and/or epoxy voids has been substantially reduced or eliminated.
A method for making an apparatus according to the invention includes a couple of basic steps. First, potting the apparatus, namely, surrounding an electrical portion of the apparatus with epoxy resin so as to encapsulate the electrical portion. Secondly, heating the apparatus according to a temperature profile. According to the invention, the temperature profile is determined as a function of (i) a minimum cure-induced stress profile associated with the epoxy resin itself, and (ii) a composite thermal transfer function associated with the apparatus. Conventional approaches assume isothermal gel/cure conditions, and, not surprisingly, do not even recognize the problem of temperature gradients within the apparatus being potted, much less provide any solutions. The present invention characterizes the apparatus being potted from a thermal perspective, determining the relationship between commanded temperature (i.e., the temperature driven in the interior of an oven, for example), and the responsive temperature at one or more different points in the interior of the apparatus where the epoxy resin is being cured. Control of the temperature where the epoxy resin is being cured is essential if the minimum cure induced stress is to be realized. The present invention enables realizing the full potential of cure induced stress profiling for non-isothermal conditions (i.e., most real-world conditions). In a preferred embodiment, the method of heating the apparatus is performed by the substep of controlling an oven in accordance with the determined temperature profile described above. The present invention has applicability toward a wide range of apparatus, and in a constructed embodiment, is applied to an ignition coil.
In a still further preferred embodiment, a method for minimizing a gel-cure cycle duration having minimized residual stress is provided. In particular, the method includes increasing the initial gel isotherm temperature, having due regard for the cure-induced stress profile for the epoxy resin material being used, as well as the thermal transfer function associated with the apparatus. That is, the heat involved in the curing of the epoxy resin material is time-temperature independent. There are therefore multiple time-temperature profiles that traverse the degree of cure path from 0% to 100%. The amount of heat introduced into the chemical action that accomplishes curing of the epoxy resin is advanced relative to, for example, manufacturer""s recommended time-temperature profiles. Gel-cure cycle times can be reduced 50% or more through the foregoing approach, thereby improving throughput and productivity, eliminating potential bottleneck situations.
A system corresponding to the inventive method is also presented.